This invention relates the field of telephony, and more particularly to a method and apparatus for detecting a dual tone alerting signal in caller ID systems.
Caller ID is the generic name for a group of services offered by telephone operating companies around the world where information about the calling party is transmitted to the called party using FSK (Frequency Shift Keyed) modulation. In the Calling Identity Delivery on Call Waiting (CIDCW) and Call Waiting Deluxe (CWD) services offered by Canadian and U.S. telephone operating companies, a signal is sent by the central office to notify the near end CPE (Customer Premise Equipment), which is already engaged in an established call, that the central office wishes to deliver calling identity information of a waited call. In North America, this signal is known as CAS (CPE Alerting Signal).
CAS is a dual tone signal used for off hook Caller ID signalling. In Europe, a similar signal known as DT-AS (Dual Tone Alerting Signal) is used in off hook and, in some networks, on hook Caller ID signalling.
In off-hook signalling CAS must be detected in the presence of speech, noise or music. Off hook signalling CAS detection must also be robust against imitation from speech, noise, or music because the CPE must be able to detect CAS throughout the entire duration of a call.
The on-hook and off-hook caller id signalling protocol is specified in Bellcore GR-30-CORE, the CPE requirements in SR-TSV-002476. In the off hook protocol, the central office mutes the far end just before the CAS is transmitted. When the near end CPE detects the CAS, it mutes the handset and checks whether there is any parallel off hook CPE. If there is no parallel off hook CPE, it acknowledges CAS reception by sending an ACK signal, which is a predefined DTMF digit, back to the central office. When the central office receives the ACK signal, it transmits the calling party information in FSK to the near end CPE, which then typically displays the information to the user.
Since the CPE is already in an established call, the near end user (the end which is to receive the calling identity information) may be speaking when CAS is transmitted from the central office. Therefore, the CAS must be detected in the presence of near end speech, noise or music. Failure to detect the CAS and reply with ACK within a defined interval is known as xe2x80x98talkdownxe2x80x99. Talkdown is undesirable because the calling information will not be delivered and the user will not receive the level of service paid for. The user might miss an important incoming call.
Since CAS can be transmitted anytime during the established call, CAS detection must also be robust against imitation from speech, noise or music from both the near end and the far end. A false detection followed by ACK is known as talkoff. Talkoff is annoying because the near end CPE sends a superfluous ACK signal and the near end CPE stays muted in anticipation of the FSK signal.
SR-TSV-002476 specifies CAS detection performance requirements, including talkoff and talkdown immunity requirements. The CAS characteristics are:
Lower tone frequency 2130 Hz+/xe2x88x920.5%
Upper tone frequency 2750 Hz+/xe2x88x920.5%
Signal level (per tone) xe2x88x9214 to xe2x88x9232 dBm (referred to 600 ohms)
Signal level differential between tones (Twist) xe2x88x926 to +6 dB
Duration 75 to 85 ms
Reject signal level (per tone) below xe2x88x9245 dBm
In the GR-30-CORE off hook protocol, the near end CPE must not send an ACK signal if there is a parallel off hook CPE. This restriction is modified by a protocol known as MEI (Multiple Extension Interworking) developed by Bellcore in conjunction with the TIA (Telecommunications Industry Association). MEI allows a CPE to ACK if all off hook CPEs are MEI compatible. MEI is described in the TIAEIA-777 standard.
In Europe, the DT-AS characteristics in off hook signalling are similar to CAS, except for minor signal level differences (xe2x88x929.78 to xe2x88x9232.78 dBm), and that there is no reject level requirement. The on hook signalling DT-AS characteristics are significantly different from CAS for signal levels (xe2x88x921.78 to xe2x88x9237.78 dBm) and duration (90 to 110 ms), again there is no reject level requirement. The specifications are in the ETSI standards ETS 300 778-1 and ETS 300 778-2. Services similar to CIDCW are offered.
FX602 Data sheet and U.S. Pat. No. 5,649,002 disclose a method wherein a determination is made whether the CAS detect signal remains in a predetermined condition at a predetermined time after the local telephone signal is suppressed. A validity signal is output when the CAS detect signal remains in the predetermined condition at the predetermined time. False detections are thereby minimized by ignoring detections caused by a signal originating at the local telephone. This method is undesirable because the speech check mutes employed are annoying to near end and far end users.
U.S. Pat. No. 5,519,774 discloses an apparatus with two channels for separately detecting each tone. Each channel includes means for comparing the signaling energy around one of the tones with the energy in a selected weighted guard band portion of the voiceband. Based on this comparison, a determination is made whether a tone is or is not present. A pulse signal formed from the coincidences of tone detection in each channel is used by timing circuitry to determine whether the dual tone alerting signal is actually present or whether speech energy caused both tones to be erroneously detected (talkoff). Characteristics of this pulse signal are analyzed to determine whether an alerting signal is present. An alerting signal is detected if parameters of these characteristics fall within determined ranges, which ranges are continuously updated based on the parameters of these signal characteristics determined from previously detected alerting signals. If, however, the signal characteristic parameters of a present potential alerting signal match those of a previously determined talkoff signal, a talkoff is presumed even when these parameters fall within ranges in which an alerting signal would otherwise be detected.
This apparatus is difficult to manufacture in analog implementation because of the stringent post limiter filter xe2x88x923 dB frequency requirements. It is expensive for DSP implementation because the stringent post limiter filters will require long word length. Additionally, the post limiter filter xe2x88x923 dB frequencies are just barely sufficient to detect tones which are at the xe2x88x920.5% or +0.5% frequency extreme of the SR-TSV-002476 requirement and leave no room for manufacturing variation and component tolerance.
According to the present invention there is provided an apparatus for detecting a dual tone alert signal in a telephone system capable of at least off hook mode operation comprising a processing channel for each tone, each processing channel comprising a bandsplit filter for extracting the tone of interest and attenuating the other tone; a comparator for comparing the amplitude of the output of said bandsplit filter with a threshold value; and an adaptive threshold generator for generating said threshold value adapted to the amplitude of the output of said bandsplit filter such that the comparator rejects speech and music imitations of the tone.
The dual tone alert signal may, for example, be a CAS signal or a DT-AS signal.
The apparatus may be selectable between xe2x80x98off hook modexe2x80x99 and xe2x80x98on hook modexe2x80x99 operations. Off hook mode is optimized for both talkoff and talkdown immunity, while on hook mode is optimized for better talkdown immunity.
Off hook mode can be used in off hook signalling CAS detection when the CPE with the invention is in the off hook state. On hook mode can be used in off hook signalling CAS detection when a parallel CPE is off hook but the CPE is in the on hook state, such as in MEI. The following description explains why on hook mode only needs to be talkdown robust.
MEI introduces the concept of the ACK Sender and Backup ACK Sender. On a per call basis, the ACK sender is the first CPE to go off hook for the call. It retains its status even if it returned on hook while the line remains off hook. The ACK sender must give up its status if a Type 3 CPE asserts its ACK Sender status.
The Backup ACK Sender is the CPE to last respond to CAS with an ACK and successfully receive FSK data. It retains its status from call to call but must give up its Backup ACK Sender status when another CPE successfully completes the CAS-ACK-FSK sequence.
Therefore an MEI compatible CPE must be able to detect CAS when itself is on hook but the line is off hook. In most CPE designs, to detect CAS when the CPE is on hook the signal must come from the Tip/Ring connection because the 4-wire side connection (to the receive pair of the telephone hybrid or speeech IC) is not operational, or the 4-wire side signal level is severely attenuated.
When the CPE is on hook (while the line is off hook), its CAS detector only needs to be talkdown immune because of the MEI CAS-ACK protocol. For the purpose of this invention, in MEI when CAS is detected:
Each off hook CPE must proceed to the on hook state not earlier than 25 ms and no later than 60 ms after the end of CAS as measured on Tip/Ring. After detecting a line HIGH state (the line voltage when the line is not terminated by any CPE), the ACK Sender (which may be on hook when the CAS came) goes off hook. The ACK sender allows the line to remain in the HIGH state for at least 8 ms but not more than 8 ms. If no line HIGH state is detected within 100 ms after going on hook, all previously off hook CPE shall return to the off hook state.
Following a CAS event, the Backup ACK Sender (which may be on hook when the CAS came) monitors the line for a line HIGH state lasting a minimum of 15 ms. Once this condition has been detected, the Backup ACK Sender shall immediately become the ACK Sender, go off hook no later than 20 ms after the start of the line HIGH state, complete the CAS-ACK handshake, and remain as ACK Sender for the remainder of the call. This situation may happen if the designated ACK Sender is not MEI compliant.
An MEI compliant CPE that is not the designated ACK Sender or the Backup ACK Sender but which is off hook at the time of the CAS, must monitor the line for a line HIGH state lasting a minimum of 30 ms. Once this condition has been detected, the CPE shall immediately become the ACK Sender, go off hook no later than 35 ms after the start of the line HIGH state, complete the CAS-ACK handshake, and remain as ACK Sender for the duration of the call. This situation can happen if the designated ACK Sender and the Backup ACK Sender are not MEI compliant.
After going off hook the ACK Sender begins transmission of the ACK no earlier than 30 ms and no later than 40 ms after the leading edge of the line HIGH voltage transition.
The on hook CPE""s (ACK Sender or Backup ACK Sender) CAS detector does not need to be talkoff robust because after CAS has been detected, the CPE must monitor the line for the line HIGH state, which can happen only if an off hook CPE also detected CAS.
However, the on hook CPE""s CAS detector must be talkdown robust. Otherwise it will miss the CAS even though the off hook CPE detected the CAS. The on hook mode can also be used to detect the ETSI on hook signalling DT-AS.
The invention also provides a method of detecting a dual tone alert signal in a telephone system capable of at least off hook mode operation comprising the steps of for each tone, extracting the tone of interest and attenuating the other tone; and comparing the level of each extracted tone with a threshold value adapted to the amplitude of the output of said bandsplit filter so as to reject speech and music imitations.